1. Field of the Invention
This invention relates to a recording medium which can be suitably used for an ink jet recording method and particularly to a recording medium excellent in absorptivity, color forming characteristic of aqueous ink, and excellent in print quality of the recorded image obtained.
Further, the present invention relates to a recording method which can provide a recorded image excellent in storability with little indoor discoloration of the image, and to an ink jet recording method which can provide the above recorded image.
2. Related Background Art
As the recording medium for ink jet recording of the prior art, there have been known: for example,
(1) one formed into a filter paper or a blotting paper by sheet making of a paper in general composed mainly of pulp to a low sizing degree;
(2) one having a coated layer containing a pigment which is porous and capable of adsorbing colored components in the ink and which shows large oil absorption, such as silica or zeolite, on a substrate as disclosed in Japanese Laid-open Patent Application No. 148585/1981.
While the recording medium (1) can be prepared at low cost and its ink absorptivity is excellent, the ink is penetrated deep into the fiber layer of the paper, whereby the color forming characteristic of the pigment in the ink is poor. Also, since the ink is absorbed along the fibers on the paper surface, a phenomenon called feathering occurs, whereby a dot does not become circular, but ragged, and the drawback is that resolution is lowered, failing to give an image of good quality.
Thus, the papers of such non-coated type have been utilized exclusively for uses with relatively lower resolution, not requiring necessarily images of high density, such as the terminals of monochromatic recording, and personal computers.
In the recording medium (2), since its ink absorbing layer is porous and uniform, one having adequate ink absorptivity as well as excellent dot shape and resolution can be obtained.
However, in a recording system which requires a color image of higher quality and higher resolution, the following requirements are further demanded also for the recording medium:
(1) it should have an ink absorbing capacity such that a dot will not flow out and will not be expanded more than is necessary even if ink droplets of multi-color may be superposed on the same spot;
(2) it should have an ink absorbing speed, and an ink dry time characteristic such that the ink droplets will not feather if smeared immediately after attachment;
(3) a recording agent in an ink received in an ink absorbing layer should be excellent in color forming characteristic;
(4) ink dots attached should be smooth at their peripheries and have a shape approximate to true sphere, etc.; and in addition, storability such as water resistance, light resistance of the recorded image obtained is required.
In U.S. Pat. No. 4,478,910 and Japanese Laid-Open Patent Application No. 230787/1984, in view of the physical properties of a pigment in an ink absorbing layer in order to obtain more excellent color forming characteristic of a dye, there is introduced a recording medium by use of silica having a specific surface area of 200 m.sup.2 /g or more or silica having an acid value of 240 mg.mol/kg or more.
Also, in Japanese Laid-open Patent Application No. 84992/1981, as a method for strengthening water resistance of an ink jet recorded image, there is described a method in which recording is performed by use of acidic/direct dyes on a recording medium having a polycationic polymeric electrolyte contained in its ink absorbing layer so that these dyes attached on the ink absorbing layer will not be flowed out even when the recorded image is dipped into water.
Light resistance of the image is concerned with the problem of discoloration and fading of the recorded image by photolysis of the dye, and it has been considered as the problem of the dye itself to date, but it has become known that a water-resistant agent as mentioned above, particularly a polycationic substance has promoted photolysis of the dye. For example, Japanese Laid-open Patent Applications Nos. 11389/1985 and 49990/1985 describe a recording medium containing a polycation with little promotion of photolysis of a dye, and also Japanese Laid-open Patent Application No. 72785/1985 describes a recording medium containing a polycation together with a UV-absorber and an antioxidant.
However, recently, a new problem concerning storability of image, particularly indoor discoloration of recorded image other than such water resistance and light resistance is becoming highlighted. The problem of indoor discoloration has been considered to be a problem caused by decomposition of a dye, but cannot be solved by the recording media known in the art.
Fading of the image, which is a problem in the prior art, is a phenomenon caused by decomposition of a dye within a recorded image by visible light, UV-ray and it will not occur at the portion which is not directly irradiated with sunlight. Also, in the place which is irradiated with sunlight, fading occurs also in an image recorded on a recording medium of any type such as so-called normal ppc paper or the recording medium (1) and (2) as mentioned above, and it has been known that fading is promoted when the recording medium contains a polycationic substance.
On the other hand, indoor discoloration as herein mentioned proceeds even when a recording medium is not directly irradiated with sunlight, and will not occur on a normal paper and a non-coated paper. Also, discoloration of an image occurs without any appreciable effect even when a polycation with little influence on light resistance as described above may be employed or even when a UV-ray absorber may be incorporated.
Fading as herein mentioned refers to the phenomenon in which chromaticity of a printed matter is lowered, while discoloration refers to a phenomenon in which chromaticity is not lowered but hue is primarily changed.
As described above, the problem of indoor discoloration as mentioned in the present invention is a phenomenon inherent in a coated paper having an ink absorbing layer, but neither its clear cause nor countermeasure has been known yet.
In addition, there is posed the problem that when such coated paper not printed is stored in a binder made of polypropylene or polyethylene for one to several months, discoloration to yellow occurs at the periphery of the paper and bringing the paper into contact with an adhesive tape or a rubber roller of a printer results in discoloration to yellow around the contact portion.
Similarly to indoor discoloration, the problem of such discoloration to yellow is inherent in the coated paper (such problem is posed in a non-coated paper). The problem has not been solved yet.